Robert Vautard at the Laboratory of Climate and Environmental Sciences (LSCE) in Gif-sur-Yvette, France, and colleagues analysed surface wind speed data from 822 sites around the world, covering the last 30 years.
They found that surface wind speeds have declined by 5 to 15 per cent across much of the northern hemisphere over this period. The slowing has been more marked for winds of over 10 metres per second than for lighter winds.
When Vautard and his team modelled the effect of vegetation increases across Europe and Asia over the period, they found this could explain most of the drop-off. "Large parts of Earth are becoming greener, partly due to efforts in Europe to maintain forests," says Jean-Noel Thépaut of the European Centre for Medium-Range Weather Forecasts in Reading, UK, who took part in the research.
Wind turbines operate most efficiently when winds are stronger, so a continuing decline in the highest surface wind speeds would lead to a significant drop in wind power production, the researchers say.
However, Lars Christian Christiansen, a vice-president at wind turbine developer Vestas, based in Randers, Denmark, says the company's own research has not shown a clear correlation between changes in surface conditions and the wind speed at mast heights.
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